1. Field of the Invention
The invention relates to optical articles including holographic recording media, in particular media useful either with holographic storage systems or as components such as optical filters or beam steerers.
2. Discussion of the Related Art
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, so-called page-wise memory systems, in particular holographic systems, have been suggested as alternatives to conventional memory devices. Page-wise systems involve the storage and readout of an entire two-dimensional representation, e.g., a page, of data. Typically, recording light passes through a two-dimensional array of dark and transparent areas representing data, and the holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index imprinted into a storage medium. Holographic systems are discussed generally in D. Psaltis et al., xe2x80x9cHolographic Memories,xe2x80x9d Scientific American, November 1995, the disclosure of which is hereby incorporated by reference. One method of holographic storage is phase correlation multiplex holography, which is described in U.S. Pat. No. 5,719,691 issued Feb. 17, 1998, the disclosure of which is hereby incorporated by reference. In one embodiment of phase correlation multiplex holography, a reference light beam is passed through a phase mask, and intersected in the recording medium with a signal beam that has passed through an array representing data, thereby forming a hologram in the medium. The spatial relation of the phase mask and the reference beam is adjusted for each successive page of data, thereby modulating the phase of the reference beam and allowing the data to be stored at overlapping areas in the medium. The data is later reconstructed by passing a reference beam through the original storage location with the same phase modulation used during data storage. It is also possible to use volume holograms as passive optical components to control or modify light directed at the medium, e.g., filters or beam steerers. Writing processes that provide refractive index changes are also capable of forming articles such as waveguides.
FIG. 1 illustrates the basic components of a holographic system 10. System 10 contains a modulating device 12, a photorecording medium 14, and a sensor 16. Modulating device 12 is any device capable of optically representing data in two-dimensions. Device 12 is typically a spatial light modulator that is attached to an encoding unit which encodes data onto the modulator. Based on the encoding, device 12 selectively passes or blocks portions of a signal beam 20 passing through device 12. In this manner, beam 20 is encoded with a data image. The image is stored by interfering the encoded signal beam 20 with a reference beam 22 at a location on or within photorecording medium 14. The interference creates an interference pattern (or hologram) that is captured within medium 14 as a pattern of, for example, varying refractive index. It is possible for more than one holographic image to be stored at a single location, or for holograms to be stored in overlapping positions, by, for example, varying the angle, the wavelength, or the phase of the reference beam 22, depending on the particular reference beam employed. Signal beam 20 typically passes through lens 30 before being intersected with reference beam 22 in the medium 14. It is possible for reference beam 22 to pass through lens 32 before this intersection. Once data is stored in medium 14, it is possible to retrieve the data by intersecting reference beam 22 with medium 14 at the same location and at the same angle, wavelength, or phase at which reference beam 22 was directed during storage of the data. The reconstructed data passes through lens 34 and is detected by sensor 16. Sensor 16 is, for example, a charged coupled device or an active pixel sensor. Sensor 16 typically is attached to a unit that decodes the data.
The capabilities of such holographic storage systems are limited in part by the storage media. Iron-doped lithium niobate has been used as a storage medium for research purposes for many years. However, lithium niobate is expensive, exhibits poor sensitivity (1 J/cm2), has low index contrast (xcex94n of about 10xe2x88x924), and exhibits destructive read-out (i.e., images are destroyed upon reading). Alternatives have therefore been sought, 1o particularly in the area of photosensitive polymer films. See, e.g., W. K. Smothers et al., xe2x80x9cPhotopolymers for Holography,xe2x80x9d SPIE OE/Laser Conference, 1212-03, Los Angeles, Calif., 1990. The material described in this article contains a photoimageable system containing a liquid monomer material (the photoactive monomer) and a photoinitiator (which promotes the polymerization of the monomer upon exposure to light), where the photoimageable system is in an organic polymer host matrix that is substantially inert to the exposure light. During writing of information into the material (by passing recording light through an array representing data), the monomer polymerizes in the exposed regions. Due to the lowering of the monomer concentration caused by the polymerization, monomer from the dark, unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting concentration gradient create a refractive index change, forming the hologram representing the data. Unfortunately, deposition onto a substrate of the pre-formed matrix material containing the photoimageable system requires use of solvent, and the thickness of the material is therefore limited, e.g., to no more than about 150 xcexcm, to allow enough evaporation of the solvent to attain a stable material and reduce void formation. In holographic processes such as described above, which utilize three-dimensional space of a medium, the storage capacity is proportional to a medium""s thickness. Thus, the need for solvent removal inhibits the storage capacity of a medium. (Holography of this type is typically referred to as volume holography because a Klein-Cook Q parameter greater than 1 is exhibited (see W. Klein and B. Cook, xe2x80x9cUnified approach to ultrasonic light diffraction,xe2x80x9d IEEE Transaction on Sonics and Ultrasonics, SU-14, 1967, at 123-134). In volume holography, the media thickness is generally greater than the fringe spacing,)
U.S. patent application Ser. No. 08/698,142 (our reference Colvin-Harris-Katz-Schilling 1-2-16-10), the disclosure of which is hereby incorporated by reference, also relates to a photoimageable system in an organic polymer matrix, but allows fabrication of thicker media. In particular, the application discloses a recording medium formed by polymerizing matrix material in situ from a fluid mixture of organic oligomer matrix precursor and a photoimageable system. A similar type of system, but which does not incorporate oligomers, is discussed in D. J. Lougnot et al., Pure and Appl. Optics, 2, 383 (1993). Because little or no solvent is typically required for deposition of these matrix materials, greater thicknesses are possible, e.g., 200 xcexcm and above. However, while useful results are obtained by such processes, the possibility exists for reaction between the precursors to the matrix polymer and the photoactive monomer. Such reaction would reduce the refractive index contrast between the matrix and the polymerized photoactive monomer, thereby affecting to an extent the strength of the stored hologram.
Thus, while progress has been made in fabricating photorecording media suitable for use in holographic storage systems, further progress is desirable. In particular, media which are capable of being formed in relatively thick layers, e.g., greater than 200 xcexcm, which substantially avoid reaction between the matrix material and photomonomer, and which exhibit useful holographic properties, are desired.
The invention constitutes an improvement over prior recording media. The invention""s use of a matrix precursor (i.e., the one or more compounds from which the matrix is formed) and a photoactive monomer that polymerize by independent reactions substantially prevents both cross-reaction between the photoactive monomer and the matrix precursor during the cure, and inhibition of subsequent monomer polymerization. Use of a matrix precursor and photoactive monomer that form compatible polymers substantially avoids phase separation. And in situ formation allows fabrication of media with desirable thicknesses. These material properties are also useful for forming a variety of optical articles (optical articles being articles that rely on the formation of refractive index patterns or modulations in the refractive index to control or modify light that is directed at them). In addition to recording media, such articles include, but are not limited to, optical waveguides, beam steerers, and optical filters. Independent reactions indicate: (a) the reactions proceed by different types of reaction intermediates, e.g., ionic vs. free radical, (b) neither the intermediate nor the conditions by which the matrix is polymerized will induce substantial polymerization of the photoactive monomer functional groups, i.e., the group or groups on a photoactive monomer that are the reaction sites for polymerization during the pattern (e.g., hologram) writing process (substantial polymerization indicates polymerization of more than 20% of the monomer functional groups), and (c) neither the intermediate nor the conditions by which the matrix is polymerized will induce a non-polymerization reaction of the monomer functional groups that either causes cross-reaction between monomer functional groups and the matrix or inhibits later polymerization of the monomer functional groups. Polymers are considered to be compatible if a blend of the polymers is characterized, in 90xc2x0 light scattering of a wavelength used for hologram formation, by a Rayleigh ratio (R90xc2x0) less than 7xc3x9710xe2x88x923 cmxe2x88x921. The Rayleigh ratio (Rxcex8) is a conventionally known property, and is defined as the energy scattered by a unit volume in the direction xcex8, per steradian, when a medium is illuminated with a unit intensity of unpolarized light, as discussed in M. Kerker, The Scattering of Light and Other Electromagnetic Radiation, Academic Press, San Diego, 1969, at 38. The Rayleigh ratio is typically obtained by comparison to the energy scatter of a reference material having a known Rayleigh ratio. Polymers which are considered to be miscible, e.g., according to conventional tests such as exhibition of a single glass transition temperature, will typically be compatible as well. But polymers that are compatible will not necessarily be miscible. In situ indicates that the matrix is cured in the presence of the photoimageable system. A useful photorecording material, i.e., the matrix material plus the photoactive monomer, photoinitiator, and/or other additives, is attained, the material capable of being formed in thicknesses greater than 200 xcexcm, advantageously greater than 500 xcexcm, and, upon flood exposure, exhibiting light scattering properties such that the Rayleigh ratio, R90, is less than 7xc3x971031 3. (Flood exposure is exposure of the entire photorecording material by incoherent light at wavelengths suitable to induce substantially complete polymerization of the photoactive monomer throughout the material.)
The optical article of the invention is formed by steps including mixing a matrix precursor and a photoactive monomer, and curing the mixture to form the matrix in situ. As discussed previously, the reaction by which the matrix precursor is polymerized during the cure is independent from the reaction by which the photoactive monomer is later polymerized during writing of a pattern, e.g., data or waveguide form, and, in addition, the matrix polymer and the polymer resulting from polymerization of the photoactive monomer (hereafter referred to as the photopolymer) are compatible with each other. (The matrix is considered to be formed when the photorecording material exhibits an elastic modulus of at least about 105 Pa. Curing indicates reacting the matrix precursor such that the matrix provides this elastic modulus in the photorecording material.) The optical article of the invention contains a three-dimensional crosslinked polymer matrix and one or more photoactive monomers. At least one photoactive monomer contains one or more moieties, excluding the monomer functional groups, that are substantially absent from the polymer matrix. (Substantially absent indicates that it is possible to find a moiety in the photoactive monomer such that no more than 20% of all such moieties in the photorecording material are present, i.e., covalently bonded, in the matrix.) The resulting independence between the host matrix and the monomer offers useful recording properties in holographic media and desirable properties in waveguides such as enabling formation of large modulations in the refractive index without the need for high concentrations of the photoactive monomer. Moreover, it is possible to form the material of the invention without the disadvantageous solvent development required previously.
In contrast to a holographic recording medium of the invention, media which utilize a matrix precursor and photoactive monomer that polymerize by non-independent reactions often experience substantial cross-reaction between the precursor and the photoactive monomer during the matrix cure (e.g., greater than 20% of the monomer is attached to the matrix after cure), or other reactions that inhibit polymerization of the photoactive monomer. Cross-reaction tends to undesirably reduce the refractive index contrast between the matrix and the photoactive monomer and is capable of affecting the subsequent polymerization of the photoactive monomer, and inhibition of monomer polymerization clearly affects the process of writing holograms. As for compatibility, previous work has been concerned with the compatibility of the photoactive monomer in a matrix polymer, not the compatibility of the resulting photopolymer in the matrix. Yet, where the photopolymer and matrix polymer are not compatible, phase separation typically occurs during hologram formation. It is possible for such phase separation to lead to increased light scattering, reflected in haziness or opacity, thereby degrading the quality of the medium, and the fidelity with which stored data is capable of being recovered